1. Field of the Invention
The present invention relates in general to a vibration cancellation apparatus for isolating floor vibrations or canceling vibrations of objective equipment thereon to affect the production yield or measurement precision of objective equipment such as semiconductor fabrication devices and electron-microscopes. Particularly the invention relates to a vibration cancellation apparatus that provides a combined action of air springs and electromagnetic actuators, in which vertical support of load is primarily provided by air spring actuators, and fine control of vibrations is primarily provided by electromagnetic actuators.
2. Description of the Related Art
Conventionally, vibration-sensitive equipment, such as electron-microscopes and semiconductor fabrication equipment whose performances are adversely affected by vibrations, have been installed on the floor, by way of some vibration isolation apparatuses such as air springs and rubber sheets. Replacing the conventional air springs and rubber-based arrangements as vibration isolation devices, high performance vibration cancellation apparatuses based on magnetic levitation have been developed.
Also a combination of air springs and an electromagnetic actuator is known for isolating floor vibrations and canceling vibrations of the objective equipment on an anti-vibration table. According to these apparatuses, both air springs and electromagnetic actuators are used to actively control levitating positions and to cancel vibrations in the vertical direction. The objective equipment is supported on an anti-vibration table whose four corners are supported by actuators which include air springs and electromagnetic actuators. Three proximity sensors and three accelerometers are used to detect table displacements and accelerations, and responding actions are quickly applied through digital controllers to the pneumatic actuators and the electromagnetic actuators.
In reviewing the existing techniques of vibration canceling control, based on a combination of air springs and electromagnetic actuators, when the center of gravity of the table moves by movement of the objective equipment placed on the anti-vibration table, two methods of leveling the anti-vibration table may be considered:
(1) To control the air pressures of the air spring actuators for leveling the table, or PA1 (2) To control the vertical positions of the table by operating the electromagnetic actuators for leveling the table. PA1 (1) Because of the slow response characteristics of air springs actuators, when the center of gravity of the table moves at high velocities, the air spring actuators are not able to respond in a timely manner to keep the anti-vibration table level; and PA1 (2) Although electromagnetic actuators are able to respond quickly for keeping the anti-vibration table level, it is necessary to increase the capacity of the electromagnetic actuators such that one of the actuators can deal with the maximum anticipated load of the objective equipment, including the weight of the table. When the center of gravity of the objective equipment moves to a corner of the table, one of the electromagnetic actuators must support all of the weight in order to keep the table level. One of the problems with this approach is that the cost of the system becomes high and the system size becomes large.
However, these approaches present the following problems.